This invention relates generally to a full frame copying device and, more particularly, to an improved optical system adapted to provide a uniform irradiance at an image plane.
In electrophotographic copiers, the areas of a charged photoconductive surface which are irradiated by a light image are discharged, the degree of discharge dependent upon the intensity of the impinging light rays. It is, therefore, desirable that the light ray intensity vary only due to the reflectance characteristics of the original document being copied rather than due to changes introduced by the imaging components. Stated in another manner, the optimum system would be one providing uniform photoconductor irradiance given a uniformly reflecting document.
Of the factors affecting relative illumination at an image plane, the most well known is the cosine (cos) variation caused by the lens wherein the illumination at an angle plane is proportional to the cos.sup.4 of the angle between the optical axis and the field beam. Thus, photoconductor irradiance decreases as radial distance from the system optical axis increases. Various approaches have been devised to compensate for this effect. Typically, in slit-scanning systems, a sheet of opaque material having a butterfly slit formed thereon is employed with the area of the slit being inversely proportional to the illlumination profile. Other similar techniques employ masks having slits of this nature integral with the lens. Still other scanning systems utilize a variable density filter in the optical path whose transmissiveness varies inversely to the cos.sup.4. Such a device is described in IBM Technical Disclosure, Vol. 14, No. 11 (April 1972).
In full frame imaging systems wherein an entire document is typically illuminated by flash illumination, efforts to compensate for cos.sup.4 light falloff have emphasized locating light sources in such a way that the document edges are illuminated to a greater degree than central areas. Two such systems are disclosed in U.S. Pat. No. 3,669,538 (Fowler) and U.S. Pat. No. 3,777,135 (Rees). These compensation techniques are fairly effective for relatively small half field angles (20.degree.) but as this field angle increases, the cos.sup.4 falloff becomes steeper and other circularly symmetric non-uniformities within the optical system contribute still further to illumination non-uniformity at the image plane. (Also, the increased edge illumination produced by these systems may not be entirely suitable for corner-registered document systems.) These non-uniformities include lens transmission variations and lens exit pupil distortion. The cumulative effect of these non-uniformities combined with the cos.sup.4 dropoff result in often severe problems in obtaining uniform exposure at the photoconductor.